In many applications it is desirable to convert a primary direct voltage to another direct voltage in an efficient manner. This problem often arises in the context of electrical equipment powered by the primary direct voltage electrical system of a vehicle. The primary direct voltage system of the vehicle is often a battery having a relatively low direct voltage, whereas the equipment to be powered may require a higher voltage or a lower voltage. The abbreviation dc (direct current) is often used to denote a direct voltage. Where the equipment to be powered requires a lower voltage at high current, a dc-to-dc converter may be used to avoid unwanted power dissipation in a voltage dropping resistor. Where an output voltage higher than the primary voltage is required, the dc-to-dc converter is desirable.
One way to implement a dc-to-dc converter is to use the primary direct voltage to energize an oscillator, and to drive the primary winding of a transformer with the output of the oscillator. The secondary voltage of the transformer at the appropriate voltage level is rectified and filtered to produce the desired direct voltage. It has been found to be more efficient to apply the primary direct voltage to a switched inductor, either with or without a transformer. In order to minimize the physical sizes of the inductor and the transformer (if used), the switching often occurs at frequencies much higher than the 50 or 60 Hertz (Hz) power line frequency. For example, dc-to-dc converters are often used in television receivers to produce the kinescope ultor direct voltage, and are switched in synchronism with the 15,750 Hz television horizontal deflection.
It is often convenient to incorporate feedback voltage regulation into a switching dc-to-dc converter to compensate for load and other variations. This is ordinarily accomplished by sensing the direct output voltage of the converter, comparing it with a reference voltage to produce an error voltage, and controlling a pulse width modulator (PWM) by means of the error voltage. The pulse width modulator, in turn, establishes the duty cycle or the ratio of the conductive time to the nonconductive time of the converter switch, which in turn establishes the amount of energy stored in the inductor for transfer to the output circuit and therefore determines the output voltage.
Constant-frequency, current-programmed dc-to-dc converters have received attention. In current-programmed converters, the switched reactance is made conductive by a clock signal, and a ramp current increases until the switched reactance is rendered nonconductive by a comparator which compares the ramp current with an error voltage derived from a comparison of the converter output voltage with a reference voltage. These converters have simpler transfer functions than pulse width modulated converters, and are therefore easier to filter for stability and reduced ripple. If the converter load increases, the voltage controlled feedback loop will tend to increase the peak current of the ramp to maintain the output voltage, which may result in destruction of high current components. A current regulation scheme is desirable for operation in conjunction with such current programmed converters.